Agriculture Reference
In-Depth Information
2.2.1
THE FORMS OF WATER IN SOILS
Water may be present in soils as a solid‚ a vapour or in its more usual liquid state.
While water in the latter state is of the greatest interest in terms of its utilisation by plant
and other soil organisms‚ the occurrence of water in other states assumes great
pedological and biological importance at certain locations and times.
At high latitudes‚ soils may remain frozen for a substantial part of the year and only
thaw in summer to shallow depths dependent on site insolation. In extreme climates such
as Antarctica‚ soils may remain frozen virtually throughout the year; as considered above‚
free water may be present at the surface only briefly in the early afternoon for a short
period during summer. Below the maximum depth of thawing lies the permafrost‚ a zone
that remains permanently frozen. With decreasing latitude‚ the permafrost occurs
progressively deeper in the profile and finally ceases to exist. However‚ surface soils in
cool temperate climates and at high elevations may freeze for variable periods during
the winter or only for periods each day‚ depending on the heat balance of the site.
The air within the larger pores of most well-drained soils is normally completely
saturated with water vapour. The amount of water-vapour-saturated air in a given soil
will depend on the total pore space and the pore size distribution. In addition‚ the amount
of liquid water present has a bearing on this‚ filling the pore spaces progressively from
small to large as the soil is wetted (Papendick and Campbell‚ 1981). Soil structure is
a major factor in determining the volume of air in a soil and thus how much water will
be present in the vapour phase. Water vapour movement in the soils of dry environments
may be of considerable importance to the biota.
2.2.2
MECHANISMS OF WATER RETENTION
Because of their polarity‚ water molecules are attracted to the mineral and organic matter
forming the walls of soil pores (adhesion or adsorption) and to other water molecules
(cohesion). Water lenses occur at contact points between solid particles and surface films
form due to capillary forces; the bonding energy of these molecules is directly related to
their distances from the surface involved (Brady and Weil‚ 1996).
The soil pore space may be partly or completely filled with water which‚ depending
on the pore-size distribution‚ will be retained within the soil with varying levels of tenacity.
However‚ the pattern of water retention in soils is extremely complex since it depends
on both the organisation of the pore space and the composition and arrangement of
the solid particles.
2.2.2.1
Soil water potential
The water potential is defined as the free or potential energy of a defined mass of
water in the soil system relative to that of a reference body of pure free water (Papendick
and Campbell‚ 1981). It is the sum of its component potentials:
